The present invention generally relates to a method and apparatus for controlling or reducing the growth of zebra mussels.
The zebra mussel, Dreissena polymorpha, is believed to have been introduced into the Great Lakes of North America in the mid 1980's. The zebra mussel is native to Europe and is thought to have been transported to the Great Lakes watershed in the fresh water ballast of a trans-oceanic ship.
Most transplanted species are rather benign and have little apparent effect on the ecology of the new host system. Some are even beneficial. However, in this case, the introduced zebra mussel, has had negative impacts on the environment and constructions of man. The zebra mussel can clog water intakes and distribution pipelines for industrial and water supply facilities while also increasing the corrosion potential of the pipelines. Further, the mussel may encrust hulls of ships and populate the interiors of watercraft engines.
Particular structures that have problems with zebra mussels infestations are water intake structures such as drinking water, industrial and power generating plants. These facilities usually have submerged pipelines that bring water to the plant from a water source such as a lake or stream. Water intakes of small diameter, 60 to 180 centimeters, and great length (over several hundred meters) are particularly vulnerable to mussel infestation. Most water intakes are disposed within a crib designed to prevent large debris from being suctioned into the intake.
Normally, water flows through the crib and intake, to a shore side pumping station. Adult zebra mussels easily clog and close the gaps in the crib and decrease the volume of water drawn to the pumping station. Depending upon the depth of the crib, a secondary problem may be increased likelihood of icing about the crib due to the zebra mussel infestation since the relative water intake velocity is drastically increased (i.e. the frazil effect).
In the water intake itself, zebra mussels reduce the amount of water the pipe can carry while increasing the friction and turbulence of the flow by increasing the roughness of the pipe surface. With larger zebra mussel infestations, additional problems occur such as the lowling of filters and pumps as clusters of mussels break off from the pipe and travel downstream. Rotting flesh of dead zebra mussels present in the water intake may increase odor, taste and bacteria levels in the water.
Other structures commonly infested by zebra mussels are industrial and power generation cooling systems. A problem with heating and cooling systems, that magnifies the harm of zebra mussels, is of scale that normally forms on heat exchange surfaces. This scale creates an opportunity for the zebra mussels to settle and attach, particularly into the cracks and crevices in the scale. At times, the mussels cause quantities of scale to separate from the heat exchange surfaces, causing fouling and other problems downstream in the system.
Current methods of treating zebra mussel infestations include mechanical, chemical or non-chemical methods. Mechanical cleaning is the most conventional treatment for reducing zebra mussel numbers. Typical mechanical cleaning agents include filters to strain the mussels from the water or mechanical "pigs" or scrubbers to scrape mussels and other debris from the interior of the pipelines. Mechanical filtering of water is effective but at certain times in the zebra mussel life span, the mussels are able to pass through the filter and survive behind the filter. Mechanical filtration or scraping cause problems in that when the filters become clogged, additional service is needed to clean the filters while the industrial plants or water feed systems may need to be taken off line for such maintenance.
Another current treatment is that of chemical killing agents such as chlorine, ozone, and other oxidizing chemicals added to the infested waters. Chlorine has been used for years in Europe to control zebra mussel infestations. A major advantage offered by chemical treatments is that they can be engineered to protect the entire water system from the water intake to the end of the system. Other types of treatment include the use of a hot water bath (i.e. thermal treatment), electric shock, and ultraviolet light.
A number of problems have persisted with the previous chemical treatment methods. The most common chemicals, chlorine and ozone, have a disadvantage in that they are toxic materials to most lifeforms. Various government agencies are looking at new discharge limits to reduce or eliminate the release of these chemicals into the water supply and environment. Further, these toxic chemicals may kill off beneficial animals. Additionally, treatment costs are not constant because various uncontrollable environmental factors affect the toxicity of the chemicals, particularly water temperature, pH, dilution and organic or inorganic compounds present in the source of water, such as the reducing agents, S.sup.2- FE.sup.2+, MN.sup.2+ and NO.sup.2-. Thermal treatment, while effective, may be prohibited by new government regulations on thermal water release, while some facilities have no capacity to generate or backflush with the large quantities of hot water needed to kill zebra mussels.
The present invention is directed to overcoming the aforementioned problems associated with prior methods of zebra mussel growth control.